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However, air tests - coupled with the quartet's disoriented behavior - quickly pointed to carbon monoxide as the culprit.

That comes as no big surprise at this time of year. The odorless gas kills 50 people a year in the state.

Carbon monoxide poisoning hospitalizes about 500 people each year in the state. It kills 2,000 people a year nationally and hospitalizes another 17,000.

The gas is generated in about 85 percent of the homes in the state, according to Kidde, a North Carolina-based manufacturer of carbon monoxide alarms and other fire safety products.

Danbury area fire marshals warned last month of a possible increase in carbon monoxide poisoning and fire emergencies, thanks to skyrocketing home heating prices.

Officials fear people will turn to less expensive, but much more dangerous, methods to heat their homes. Those methods include anything from the use of space heaters to leaving the kitchen stove open. "We haven't seen it yet but we do have to be careful and use caution,"

In July, Connecticut became the fourth state to approve a law requiring carbon monoxide detectors in new houses. The law applies to houses built after Oct. 1, 2005.

"People should install CO detectors," Rickert said. "They should check their heating appliances and prevent blockages in their chimneys and furnaces."

In fact, Wednesday's incident was caused by a bird's nest that clogged a chimney, causing toxic gas from the boiler under the apartment to enter Emilia Pena's apartment.

While the four victims were put in ambulances, firefighters searched for the gas smell - and quickly learned residents weren't imagining it. A Yankee Gas crew discovered a leaky gas line outside the complex, Danbury Fire Chief

Hartford Hospital has a 20-foot-wide hyperbaric chamber that provides patients with huge hits of pure oxygen that help cleanse the blood of toxins. It can treat up to 10 carbon monoxide victims at the same time.

A hospital spokesman would say only that Pena is being treated for carbon monoxide poisoning. But the hospital's Web site says people exposed to carbon monoxide get three 90-minute sessions in the pressurized chamber over 24 hours to get the toxins out.

Carbon monoxide kills by reducing the blood's ability to carry oxygen. According the
U.S. Environmental Protection Agency
's New England office, symptoms of carbon monoxide poisoning include dizziness, fatigue, nausea and irregular breathing.

Those symptoms have
Ethel Lee
worried. She lives in the apartment next to the where the victims collapsed - and high levels of carbon monoxide were found in her apartment as well.

She wasn't home Wednesday morning, but heard the news after coming home in the evening. Lee said she had complained to the property manager about the recurring gas smell.

"I haven't been feeling well and my heart has been pounding," Lee said Wednesday before leaving to go to Danbury Hospital to see a doctor.